Decades after industrial waste dumping turned part of Southern California's seafloor into a toxic hot spot, scientists have dredged up a mystery.

Chemicals fouling the ocean off the Palos Verdes Peninsula seem to be going away without being cleaned up.

Samples taken from the sediment suggest more than 100 metric tons of the banned pesticide DDT and industrial compounds known as polychlorinated biphenyls, or PCBs, have vanished from one of the country's most hazardous sites, almost a 90% drop in just five years.

Scientists are at a loss to explain the decline across the 17-square-mile site, which sits about 200 feet below the ocean surface and two miles off the Los Angeles County coast. The compounds break down very slowly. They have accumulated in the food web over decades, made some sport fish unsafe to eat and, until recently, rendered bald eagles on Santa Catalina Island unable to reproduce.

In response to the discovery, the U.S. Environmental Protection Agency has suspended its planned cleanup efforts and ordered a new round of tests to be completed over the next year. Researchers began collecting samples from the seafloor last month.

"The precipitous drop needs to be explained," EPA Regional Administrator Jared Blumenfeld said. "The question we're answering is: Is the DDT still there?"

From the late 1940s to the early 1970s, Montrose Chemical Corp. dumped millions of pounds of DDT from its plant near Torrance into Los Angeles County sewers. The wastewater, also contaminated with PCBs from other industrial sites, emptied into the ocean off White's Point. In what became the world's largest DDT dump site, 110 metric tons of DDT and 10 tons of PCBs settled on the ocean floor, lingering in the mud for decades.

A legal battle over the pollution ended in 2000 with Montrose, other manufacturers and the county sanitation agency paying $140 million in settlements. The funds would pay for cleanup, habitat restoration and education programs for anglers at risk of eating contaminated fish.

The newest delay has frustrated environmentalists, who have criticized the EPA for moving too slowly on the cleanup. The agency added the most contaminated portion of the Palos Verdes Shelf to its list of Superfund sites in 1996.

Yet even critics said the EPA has no choice but to redo its measurements.

"We need more information before we go out and celebrate nature taking care of itself," said James Alamillo, urban programs manager at the environmental group Heal the Bay. "Let's put that on hold until we fully understand what's happening."

Scientists have several theories. The chemicals could be escaping into the water. Or they may have been covered by clean sediment deposited by the Palos Verdes Peninsula's landslide-prone coastline.

EPA officials are nearly certain some of the polluted material is being swept off the shelf into waters more than 3,000 feet deep. They also say the compounds are shedding chlorine to become less toxic.

"We know it is breaking down, we just don't know the mechanism," said Judy Huang, project manager for the Palos Verdes Shelf Superfund site.

Among those suspicious of the EPA's measurements is Chris Sherwood, an oceanographer at the U.S. Geological Survey Woods Hole Science Center who has studied the Palos Verdes Shelf sediment since the mid-1990s.

"My guess is it's a little bit of a statistical anomaly," he said. "If we sample it again and it's just as clean, then we have a scientific mystery on our hands."

The EPA chose a $35-million plan in 2009 to clean the hard-to-reach site by covering the most contaminated seafloor with clean sediment. The idea was to isolate the chemicals from the ocean environment, over time reducing levels in marine mammals, birds and fish.

Before beginning the cleanup, however, scientists detected the sudden decline in pollution from levels measured in the last round of tests, in 2004. Concentrations were already below the targets the EPA hoped to achieve through the cleanup.

The agency reviewed its data collection, statistics and lab work over the next two years and concluded the measurements were valid. The new sampling, which will cost $3.5 million, will ensure money is not wasted on an unnecessary cleanup, Blumenfeld said.

Once the results are available, probably by the end of 2014, the agency will decide whether to proceed with the cleanup or make other plans.

The seafloor, however, is just one indicator of the chemicals' presence in the environment.

"The most important information is whether fish are still contaminated," said Guang-yu Wang, senior scientist at the Santa Monica Bay Restoration Commission. "For now, all the data are showing they are still unsafe to eat."

Over the next year or two the EPA expects results from separate studies of the contamination of fish tissue and the water column as well as the movement of the sediment and the dechlorination of DDT. That data could provide a clearer picture of where the chemicals are lurking.